1. Field
The invention relates to a scheme for measuring the impedance of a load, e.g., an audio load, coupled to an amplifier.
2. Background
Portable electronic devices such as cellular phones, notebook computers, and/or portable media players commonly incorporate an audio output device, e.g., a miniature speaker. Miniature speakers are typically not very robust, i.e., they can readily fail due to over-heating if driven by excessively large voltages. To prevent such failure, a device incorporating the speakers may also include circuitry for detecting the load impedance, e.g., the resistance of the miniature speaker. Upon detecting the load impedance, the device may take various steps to prevent failure of the load, e.g., by limiting the maximum drive voltage applied to such load, and/or by using the detected impedance directly as an indication of load temperature to determine whether the load should be driven or not.
To detect load impedance, a sense resistor may be coupled in series with the load, and the load impedance may be derived from measurements of the voltage drops across the load and the sense resistor, combined with a priori knowledge of the resistance of the sense resistor. Typically, it is desired to make the sense resistor much smaller than the load to avoid unnecessary power dissipation. Accordingly, the gain characteristics of a first signal path used to process the load voltage drop may be very different from the gain characteristics of a second signal path used to process the sense resistor voltage drop, due to the significantly different voltage swings expected. The large difference between the signal path gains may degrade the accuracy of the load impedance measurement, as it may be difficult to obtain accurate matching between the two signal paths and to minimize relative gain errors.
It would be desirable to provide techniques to improve the accuracy of load impedance measurement given the design constraints of such a system.